A deposit is defined as a group of occurrences large enough to be worked (also called a mine). A find is described as a single occurrence.
Primary deposits refer to gems found in their original location. The yield is generally low due to the fact that many tons of non-gem bearing material have to be excavated.
Secondary deposits refer to gems which have been transported by the actions of wind, rain and flowing water.
Fluvial deposits are created by rivers, marine deposits by the sea and aeolian Deposits by the wind.
The distribution of gems around the world is irregular. Gem deposits occupy only a tiny proportion of the earth’s crust. Some of the more favoured regions are:
The gem-rich metamorphosed limestones of Upper Mayanmar (including the “Mogok Stone Tract”) renowned for Rubies, Sapphires and Spinels.
The alluvial deposits of Sri Lanka, Vietnam, Thailand and Madagascar and the Minas Gerais and Minas Novas districts of Brazil.
Most gemstones are discovered by accident. There is no systematic approach used for Colored Gemstones due to the lack of capital. Exceptions being Australia, where the mining of the Sapphires is more mechanized and systematic.
Recovery is accomplished invariably by simple means without the use of modern techniques or scientific basis. In many cases, the methods are very primitive and have remained unchanged virtually for 2,000 years.
They consist of:-
- Collecting gems from the surface, from dry river beds, or rock fissures.
- Sinking shafts into the ground sometimes up to 30ft deep.
- Panning rivers.
- Driving short tunnels into the sides of hillsides (used for mining Ruby, Sapphire in Mayanmar and Emerald in Colombia)
- Open-cast mining (i.e Ammolite)
- Using powerful jets of water, when available, to loosen the gem material from the overburden.
- Digging pits in ancient river beds to reach the “Gem Gravels”, (i.e. Sri Lanka, Mayanmar, Thailand).
- Terrace mining (i.e The Chivor Mine in Colombia).
- Underground mining, which is expensive and can only be justified if a significant vein is located.